PROJECT SUMMARY In our prior studies supported by NIH funding we identified the role of the bone marrow (BM) microenvironment in conferring growth, survival, and drug resistance in multiple myeloma (MM) cells. Importantly, we have successfully translated multiple novels agents (bortezomib, carfilzomib, lenalidomide, and pomalidomide) targeting these interactions from the bench to the bedside and FDA approval for treatment of MM. However, MM remains incurable in many cases despite novel therapies, suggesting the need for further identification of factors in the host-MM BM microenvironment that mediate tumorigenesis and drug resistance. Our studies provided the first evidence that plasmacytoid dendritic cells (pDCs) in the BM microenvironment both mediate characteristic immune deficiency in MM; as well as promote tumor cell growth, survival, and drug resistance. Specifically, we showed increased numbers and more frequent localization of pDCs in MM patient BM than normal BM. The functional significance of increased numbers of pDCs in MM BM is evident from our observations that pDCs: are relatively resistant to novel and conventional therapies; protect tumor cells from therapy-induced cytotoxicity; as well as promote tumor growth and survival. Aberrant pDC function is evidenced in their interactions not only with MM cells, but also with other immune effector T cells and NK cells, thereby suppressing immune responses in MM. Based on these findings, we hypothesize that directly targeting pDCs and/or pDCs interactions with MM and immune effector cells in the MM BM milieu will enhance both anti- tumor immunity and cytotoxicity. The current proposal aims to target pDCs and pDC-MM-T-NK cell interactions in novel therapeutic strategies for MM with the goal of restoring anti-MM immunity, enhancing MM cytotoxicity, overcoming drug-resistance, and improving patient outcome. We propose to utilize two distinct, yet interconnected and complementary, approaches: 1) Depletion of pDCs in the MM BM milieu (Aim 1) using a novel therapeutic strategy directed specifically against dysfunctional pDCs; and 2) Restoration of pDC immune function by triggering pDC maturation and/or blocking the immune checkpoints mediating pDC-T cell, pDC-NK cell, and pDC-MM cell interactions (Aim 2). To accomplish these goals, we will pursue the following Specific Aims: Specific Aim 1: To investigate pDCs-depletion as a novel therapy in MM. (1a) To conduct a Phase I/II clinical trial of novel agent SL-401 to deplete dysfunctional pDCs. (1b) To pre-clinically examine efficacy of anti-MM therapies with pDCs depletion. Specific Aim 2: To restore pDCs immune function by inducing pDC maturation and/or blocking the immune checkpoints mediating pDC-T cell, pDC-NK cell, or pDC-MM cell interactions. The current proposal is therefore innovative, since it will for the first time translate our research targeting MM-pDCs to the bedside and clinical trials, as well as provide the pre-clinical basis for future combination novel immune-based therapies.